(1) Field of the Invention
The present invention relates to an antireflection coating for a potassium chloride substrate of a transparent optical component such as a window and a lens which are obtained using potassium chloride.
(2) Description of the Prior Art
Materials such as ZnSe and GaAs are used as substrate materials for transparent optical components such as a window, a lens and a beam splitter for a high-power carbon dioxide gas laser. ZnSe is a transparent material at wavelengths of 10.6 .mu.m and 0.6328 .mu.m, has good moisture resistance, and yields large crystals. Although ZnSe has the above advantages, it is expensive and causes a large optical distortion upon high-power irradiation. In addition to these disadvantages, a toxic gas such as H.sub.2 Se gas must be used at the time of crystal growth, and a toxic selenium gas is generated during the crystal finish process. Thus, ZnSe cannot provide suitable safety guarantees.
GaAs is a transparent material at a wavelength of 10.6 .mu.m. The heat conductivity of GaAs is three times that of ZnSe, and GaAs has good moisture resistance. Although GaAs has the above advantages, it also has disadvantages in that a GaAs crystal having a diameter of more than 8 cm is not available, and that GaAs does not transmit an He-Ne laser beam (having a wavelength of 0.6328 .mu.m) for visible light superposition therethrough. In addition to these disadvantages, GaAs is expensive and has a large optical distortion. Furthermore, arsenic (As) contained in GaAs is a toxic element, so that safety measures must be provided during the manufacturing process.
KCl is transparent at wavelengths of both 10.6 .mu.m and 0.6328 .mu.m, nontoxic, and low in cost. KCl also has a small optic distortion. Although KCl has such several advantages, it is not used as a substrate of a commercially available transparent optical component (e.g., window and lens) since KCl is hygroscopic. If a bare KCl is used in a high humidity, it cannot withstand the humidity over a long period of time. However, if an antireflection coating for a KCl substrate is developed wherein the antireflection coating has a substantially zero reflectance at a wavelength of 10.6 .mu.m, becomes transparent at a wavelength of 0.6328 .mu.m, and has good moisture resistance, a satisfactory KCl window and lens can be obtained. However, at present, such an antireflection coating having good optical characteristics and good moisture resistance is not available.
At present, antireflection coatings for the commercially available KCl windows and KCl lenses are made of NaF single layer. The refractive index of NaF is 1.23, which is very close to the square root (.sqroot.1.45 =1.20.sub.4) of the refractive index of KCl. Therefore, it is expected that an ideal reflectance of 0.05% can be obtained if an NaF antireflection coating having an optical thickness nd=.lambda./4=2.65 .mu.m is deposited on a KCl substrate. The present inventors purchased from a US manufacturer a KCl window (having a diameter of one inch and a thickness of 5 mm) having antireflection NaF coatings on two sides thereof, and a reflectance of this KCl window was measured at a wavelength of 10.6 .mu.m. The reflectance was about 0.4%, and an absorption was about 0.3%. It was thus found that this KCl window had sufficient optical characteristics for practical use. However, when the KCl window was subjected to an environmental test at a temperature of 45.degree. C. and a relative humidity of 95%, the NaF coatings peeled off from the KCl substrate in six hours. Therefore, this NaF antireflection coating is unsatisfactory in practice from the viewpoint of moisture resistance.
Any other single layer antireflection coating excluding the NaF coating does not satisfy the above conditions of the antireflection coating. Two- or three-layer antireflection coatings must be considered. The required conditions of a material of the antireflection coatings are that it be substantially insoluble to water, and transparent at wavelengths of 10.6 .mu.m and 0.6328 .mu.m. In addition to these conditions, the antireflection coatings must provide good adhesion to the substrate and be made of a material which is amorphous and when formed into a thin coating does not form pinholes. Chalcogenide glass such as arsenic trisulfide (As.sub.2 S.sub.3) and arsenic triselenide (As.sub.2 Se.sub.3) and thorium tetrafluoride (ThF.sub.4) are promising materials.